The invention relates to a photometry power supply for automatic electronic flash, and more particularly to a power supply for a photometric circuit which may be used in an automatic electronic flash in which photometry of reflected light from an object being photographed which is illuminated by flashlight from a flash discharge tube causes the emission of the flashlight to be interrupted automatically whenever a proper photometric value is reached.
A photometric circuit used in an automatic electronic flash requires the supply of an operating voltage thereto in order to initiate a photometric operation at the time when reflected light from an object being photographed is incident thereon. To meet this need, the prior art provided a feeding capacitor which is separate from a main capacitor, or an impedance element which is connected in series with a flash discharge tube. A conventional power supply for a photometric circuit used in an automatic electronic flash will be described first.
FIG. 1 illustrates an automatic electronic flash disclosed in Japanese Patent Publication No. 38,413/1973 including a flashlight emission circuit 1. The circuit 1 includes a trigger switch 2 which may be closed to trigger a flash discharge tube 3. Thereupon, the charge stored on a main capacitor 4 discharges through the tube to emit flashlight, and the amount of light reflected from an object being photographed is determined by a photometric circuit 7 comprising a photoconductor 5 and an integrating capacitor 6. When the photometric value which is determined reaches a given value, a relay discharge tube 8 is rendered conductive to energize an emission control element 9, which operates to cease the emission of flashlight from the discharge tube 3. The photometric circuit 7 utilizes a capacitor 11 which is charged to a constant voltage by a supply circuit 10, as its power supply, and is fed therefrom through a switch 12 which is closed to initiate the photometric operation in synchronized relationship with the closure of the trigger switch 2.
However, in the described electronic flash, the power supply for the photometric circuit requires the provision of its devoted feeding capacitor 11, precharged by the supply circuit 10 associated with the electronic flash, independently from the main capacitor. Also, the switch 12 which initiates the photometric operation in interlocked relationship with the trigger switch 2 must be separately provided, resulting in an increased size and complex construction of the electronic flash. In addition, the arrangement is subject to the occurrence of a malfunctioning in response to a trigger noise.
FIG. 2 illustrates another arrangement for an automatic electronic flash disclosed in Japanese Patent Publication No. 47,327/1977. In this arrangement, when a flash discharge tube 13 is triggered and emits flashlight by allowing a discharge of a main capacitor 14 therethrough, a photometric circuit 15 is fed from an associated supply circuit 16 including a feeding capacitor 17, which discharges through a path including the discharge tube 13, Zener diode 18 and resistor 19 whenever the discharge tube 13 is triggered into conduction, thereby developing a voltage of a constant magnitude across the Zener diode 18 which is utilized as an operating voltage by the photometric circuit 15. When the photometric circuit 15 is fed from the Zener diode 18, there occurs a flow of photocurrent in accordance with the incidence of reflected light from an object being photographed upon a photoelectric transducer element 20 to thereby charge a capacitor 21. When the voltage across the capacitor 21 reaches a given value, UJT (unijunction transistor) 22 conducts, supplying a trigger pulse to the gate of a thyristor 23 to render it conductive, thereby terminating the emission of flashlight from the discharge tube 13.
The described electronic flash is again provided with the feeding capacitor 17 which is charged separately from the main capacitor 14 in order to feed the photometric circuit 15. Since the voltage across the capacitor 17 is utilized as the power supply for the photometric circuit 15, an increased size and a complex arrangement of the resulting network are unavoidable. In addition, since the feeding capacitor 17 discharges as the discharge tube 13 emits flashlight, to develop the supply voltage across the Zener diode 18 after the initiation of emission of flashlight from the discharge tube 13, it follows that the supply voltage to the photometric circuit 15 disadvantageously has a very poor rising end.
FIG. 3 shows a further automatic electronic flash disclosed in Japanese Utility Model Publication No. 253/1976. In this arrangement, as a flash discharge tube 24 is triggered into conduction by a trigger circuit 25, the charge stored across a main capacitor 27 which has been previously charged from a supply battery 26 discharges through a series combination of the flash discharge tube 24 and a resistor 28. The voltage developed across the resistor 28 charges another capacitor 29 to enable a photosensitive element 30 to be responsive to reflected light from an object being photographed which is illuminated by flashlight produced by the flash discharge tube 24, to produce a current in accordance with the amount of such incident light. This current is fed to an integrating circuit 31, thus operating it. When an integral formed by the integrating circuit 31 reaches a given value, a signal generator 32 and an associated driver 33 are operated to drive a discharge element 34, which is effective to terminate the emission of flashlight from the discharge tube 24.
In the arrangement of FIG. 3, the resistor 28 which is utilized to derive an operating voltage for the photometric circuit by charging the capacitor 29 is connected in series with the flash discharge tube 24, thus representing a loss in the emission of flashlight from the discharge tube 24, thus causing a reduction in its luminous efficiency. In addition, as in the arrangement of FIG. 2, the supply voltage has a poor rising characteristic at the initiation of the photometric operation since the capacitor 29 begins to be charged after the emission of flashlight from the discharge tube 24.
It will be seen from the foregoing that the power supply for the photometric circuit utilized in a conventional automatic electronic flash employs either a capacitor, separate from a main capacitor and which is previously charged by a power supply associated with the electronic flash or an impedance element such as a resistor connected in series with a flash discharge tube, so that the supply voltage exhibits a poor rising characteristic or a reduction in the luminous efficiency of the flash discharge tube is caused. In addition, the circuit arrangement cannot be made as compact and simple as desired.